(Nanowork News) The sparkle on brightly colored butterfly wings does not come from pigment. Rather, it is the photonic crystals that are responsible for color reproduction. Their periodic nanostructures allow certain wavelengths of light to pass through and reflect others. This makes the wing scales, which are actually transparent, look very colorful.
Fabricating artificial photonic crystals for visible wavelengths has been a major challenge and motivation for research teams since theorists predicted it 35 years ago.
“Photonic crystals have a variety of applications. They have been used to develop more efficient solar cells, innovative optical waveguides and materials for quantum communications. However, manufacturing it required very painstaking work,” explains Dr. Gregor Posnjak.
The physicist is a postdoctoral researcher in the research group of Professor Tim Liedl at LMU, and his research is funded by the “e-conversion” Cluster of Excellence and the European Research Council. Using DNA nanotechnology, the team developed a new approach for fabricating photonic crystals.
Their results have now been published in the journal. science (“Diamond lattice photonic crystals assembled from DNA origami”).
Diamond structure composed of DNA strands
Unlike lithographic techniques, the LMU team used a method called DNA origami to design and synthesize building blocks that then self-assemble into specific lattice structures.
“It has long been known theoretically that diamond lattices have the optimal geometry for photonic crystals. In diamonds, each carbon atom is bonded to four other carbon atoms. Our challenge was to scale the structure of a diamond crystal 500 times so that the space between the building blocks matches the wavelength of light,” explains Tim Liedl. “We increased the periodicity of the lattice to 170 nanometers by replacing individual atoms with larger building blocks, in our case through DNA origami,” said Posnjak.
Perfect molecular folding technology
What sounds like magic is actually the specialty of the Liedl Group, one of the world's leading research teams in DNA origami and self-assembly. To do this, scientists use a long loop-shaped strand of DNA (consisting of about 8,000 bases) and a set of 200 short DNA staples.
“The latter controls the folding of long DNA strands into almost any shape. It's similar to an origami expert who folds pieces of paper into complex objects. Clamps are therefore a means of determining how DNA origami objects combine to form the desired diamond lattice,” says the LMU postdoc.
The DNA origami building blocks form crystals about 10 micrometers in size, which are deposited on a substrate and then transferred to a collaborating research group at the Walter Schottky Institute at the Technical University of Munich (TUM). The team led by Professor Ian Sharp (also funded by the “e-conversion” Cluster of Excellence) was able to deposit individual atomic layers of titanium dioxide on all surfaces of DNA origami crystals.
“The DNA origami diamond lattice serves as a scaffold for titanium dioxide, which determines the photonic properties of the lattice due to its high refractive index. “After coating, our photonic crystals do not allow ultraviolet rays with a wavelength of around 300 nanometers to pass through, but rather reflect them,” Posnjak explains. The wavelength of reflected light can be adjusted through the thickness of the titanium dioxide layer.
DNA origami could improve photonics
For photonic crystals operating in the infrared range, classical lithographic techniques are suitable, but laborious and expensive. Lithographic methods in the wavelength range of visible and UV light have not been successful to date.
“As a result, the relatively facile fabrication process using self-assembly of DNA origami in aqueous solution provides a powerful alternative to cost-effectively mass-produce structures of desired sizes,” says Professor Tim Liedl.
He is confident that the unique structure with large chemically processable pores will stimulate further research, such as in the field of energy harvesting and storage. In the same issue of Science, a collaboration led by prof. Petr Šulc from Arizona State University and TUM presents a theoretical framework for designing diverse crystalline lattices in uneven colloids and utilizes DNA origami building blocks to form pyrochlore lattices that could potentially also be used in optical applications. We show experimentally how to do it.